In mammalian cells and organs, oxygen plays a vital role in bioenergetics, primarily through oxidative phosphorylation in mitochondria. In these organelles, an electron transport chain establishes a proton gradient across the inner membrane that drives phosphorylation of ADP to ATP. Oxygen acts as the final electron acceptor in the chain, to form water. However, within the mitochondria, the electron transport system (ETS) has also been well-studied as a source of reactive oxygen species (ROS) production with superoxide radicals forming spontaneously at complexes I, II, and III via the swift addition of leaking electrons to an incompletely reduced oxygen. During ischemia, mitochondrial ROS is increased from several sites, largely a result of oxidative damage to ETS and to enzymes in the matrix. Upon reperfusion following ischemia, a burst of ROS from the ETS is a well-characterized phenomenon that can have deleterious effects.
Normally sufficient O2 is provided by circulated blood, but there are a number of clinically relevant conditions in which acute or chronic ischemia results from inadequate blood flow. When oxidative stress is prolonged or of high enough magnitude in disease settings, cells respond in a manner that results in a maladaptive phenotype. Overwhelming levels of oxidative stress in disease states leads to alterations in myocyte mitochondrial function and polarity, cell death, and hyper-activation of neutrophils/macrophages with subsequent fibrosis.
Ischemia is a vascular disease involving an interruption in the arterial blood supply to a tissue, organ, or extremity that, if untreated, can lead to tissue death. It can be caused by embolism, thrombosis of an atherosclerotic artery, or trauma. Venous problems like venous outflow obstruction and low-flow states can cause acute arterial ischemia. Many tissues can be impacted by ischemic conditions, including the heart, large and small intestines, brain, limbs, kidneys, liver, etc.
Since oxygen is carried to tissues in the blood, insufficient blood supply causes tissue to become starved of oxygen. In the highly aerobic tissues of the heart and brain, irreversible damage to tissues can occur in as little as 3-4 minutes at body temperature. The kidneys are also quickly damaged by loss of blood flow. Tissues with slower metabolic rates may undergo irreversible damage after 20 minutes. In the absence of oxygen, ATP production in a mammalian cell is considerably less efficient, and the balance of reactants is undesirably skewed to an excess of CO2. Without immediate intervention, ischemia may progress quickly to tissue necrosis and gangrene within a few hours.
Restoration of blood supply to ischemic tissues can cause additional damage known as reperfusion injury that can be more damaging than the initial ischemia. Reintroduction of blood flow brings oxygen back to the tissues, causing a greater production of free radicals and reactive oxygen species that damage cells. It also brings more calcium ions to the tissues causing further calcium overloading and can result in potentially fatal cardiac arrhythmias and also accelerates cellular self-destruction. The restored blood flow also exaggerates the inflammation response of damaged tissues, causing white blood cells to destroy damaged cells that may otherwise still be viable.
Among ischemic conditions, acute coronary syndromes (ACS) result from acute obstruction of a coronary artery. Consequences depend on degree and location of obstruction and range from unstable angina to non-ST-segment elevation MI (NSTEMI), ST-segment elevation MI (STEMI), and sudden cardiac death. In the US, about 1.5 million myocardial infarctions (MIs) occur annually, resulting in death for 400,000 to 500,000 people. These syndromes usually occur when an acute thrombus forms in an atherosclerotic coronary artery. Initial consequences vary with size, location, and duration of obstruction and range from transient ischemia to infarction.
Ischemic, but not infarcted, tissue has impaired contractility and relaxation, resulting in hypokinetic or akinetic segments; these segments may expand or bulge during systole (called paradoxical motion). The size of the affected area determines effects, which range from minimal to mild heart failure to cardiogenic shock. Some degree of heart failure occurs in about two thirds of hospitalized patients with acute MI, which is myocardial necrosis resulting from abrupt reduction in coronary blood flow to part of the myocardium. Infarcted tissue is permanently dysfunctional; although there is a zone of potentially reversible ischemia adjacent to infarcted tissue.
Over the past decades, research and innovation have enabled advances in preventative, pharmacologic, and surgical strategies to greatly augment the clinician's ability to treat once devastating acute and chronic cardiac events. Stemming from these accomplishments, a more recent wave of exploration into cardiac tissue regeneration and angiogenesis has yielded exciting results in preclinical models and early clinical trials. While these myocardial repair strategies for cardiac injury possess great promise and popularity, it remains critical to pioneer alternative and uncharted pathways for the treatment of myocardial injury.
In addition to in vivo ischemic situations, the removal, storage, and transplantation of a solid organ from a donor profoundly alters the homeostasis of the interior milieu of the organ. These effects manifest in the degree to which the return of normal organ function is delayed or prevented after transplantation is completed. The injury an organ sustains during recovery, preservation, and transplantation occurs primarily as a result of ischemia and hypothermia. Techniques for organ preservation serve to minimize this damage to promote optimal graft survival and function.
During ischemia and organ preservation, the glycolytic pathway is shunted to lactate production, as the Krebs tricarboxylic acid cycle (TCA) cycle and mitochondrial respiration are impaired. Mitochondrial dysfunction is responsible for most of the changes in cellular energy associated with ischemia and organ preservation. Much of the injury to transplanted organs occurs not during ischemia, but during reperfusion. This finding has led to many advances in organ preservation aimed at preventing this type of injury. Furthermore, some of the events that occur during reperfusion may result in enhanced immunogenicity of the graft.
The present invention provides novel methods for correcting the metabolic imbalances that result from ischemia. These compositions and methods are of great clinical interest for in vivo and ex vivo methods of treatment.